Substrate Glass Particle Size Testing: Methods, Standards, and Applications

Substrate glass particle size testing (granularity detection) is a critical quality-control step in the production of display glass substrates, glass frits, and fine glass powders used throughout the electronics industry. Accurate characterization of particle size distribution helps manufacturers control melting behavior, surface quality, sintering performance, and final panel yield, while ensuring compliance with international standards such as ISO 13320 and GB/T 19077. This article explains the key testing parameters, sample types, analytical methods, and typical applications of particle size analysis for substrate glass materials.

Technical Overview

Substrate glass refers to high-purity, ultra-flat glass sheets—typically alkali-free aluminosilicate or borosilicate compositions—that serve as carrier substrates in TFT-LCD and OLED display manufacturing, as well as the glass powders, frits, and cullet used to produce these substrates and related electronic components. Particle size testing (granularity detection) for substrate glass covers two related areas: measurement of the particle size distribution (PSD) of glass powders and frits, and detection or counting of particles on the surfaces of finished glass substrates.

Particle size distribution directly influences process performance and final product quality. In glass melting, powder fineness affects melting efficiency, homogenization, and the formation of defects such as seeds, stones, and cords. In glass frit applications—such as OLED encapsulation seals, hermetic feedthroughs, and thick-film pastes—the PSD governs sintering shrinkage, seal porosity, and adhesion strength. On finished substrates, surface particles are a leading cause of pixel defects, photolithography faults, and yield loss in panel fabrication. Consequently, both powder producers and display manufacturers rely on rigorous granulometric control.

Key metrics in particle size characterization include percentile diameters (D10, D50, D90), mean diameters such as the volume moment mean D[4,3] and surface moment mean D[3,2], distribution width or span, cumulative undersize/oversize percentages, fine-particle content below a defined cutoff, and the number density and size distribution of surface particles on glass substrates.

Testing Samples

Glass Powder and Frit Raw Materials

Ground, milled, or spray-dried glass powders and frits destined for substrate production or electronic sealing. These are the most common samples for laser diffraction and image-based PSD analysis, and typically require representative sampling through rotary riffling or coning-and-quartering to eliminate segregation bias.

Classified and Milled Substrate Glass

Crushed substrate glass, glass cullet, and air-classified or jet-milled powders used as recycled raw material, filler, or specialty additive. Testing verifies that milling and classification operations meet the specified particle size envelope, including oversize and fines fractions.

Glass Pastes, Slurries, and Dispersions

Glass frit pastes and screen-printable or dispensable slurries used in sealing, passivation, and packaging processes. Particle size analysis of these suspensions confirms dispersion stability, prevents screen clogging, and ensures that no agglomerates or oversized particles exceed the application limits.

Finished Substrate Surfaces

Cleaned glass substrates intended for display or semiconductor use. Surface particle inspection quantifies the number, size, and location of particles on the glass surface, typically expressed as particles per unit area above defined size thresholds, supporting cleanliness qualification before panel fabrication.

Testing Items

Typical testing items for substrate glass granularity detection include:

• Particle size distribution (volume-, number-, or surface-weighted basis)
• Percentile diameters: D10, D50 (median), D90, and D99
• Mean diameters: D[4,3] (volume moment mean) and D[3,2] (Sauter mean diameter)
• Distribution width parameters, such as span = (D90 − D10) / D50
• Cumulative undersize or oversize percentages at specified cutoffs
• Fine fraction content (for example, particles below 1 µm or 2 µm)
• Oversize and foreign particle detection (sieve residue, agglomerates, hard contaminants)
• Particle morphology parameters via image analysis (circularity, aspect ratio)
• Surface particle counts, size classification, and positional mapping on finished substrates
• Comparative batch-to-batch consistency and specification conformity assessment

Testing Methods

Method selection depends on the expected size range, sample state (dry powder, suspension, or solid surface), required basis of measurement, and applicable standards. Common approaches include:

Laser Diffraction

Laser diffraction, performed in accordance with ISO 13320 and GB/T 19077, with related practices described in ASTM standards, is the primary method for glass powders and frits. It covers an approximate range of 0.1 µm to 3,000 µm using Mie or Fraunhofer optical models, supports both wet and dry dispersion, and delivers rapid, reproducible volume-based distributions. It is the reference technique for routine QC of substrate glass powders.

Dynamic Light Scattering (DLS)

Dynamic light scattering per ISO 22412 and GB/T 19627 measures hydrodynamic diameters of submicron and nanoscale glass fines in suspension, typically from about 1 nm to several micrometers. It is used to characterize colloidal fines, polishing slurries, and ultrafine fractions that fall below the practical resolution of laser diffraction.

Static and Dynamic Image Analysis

Image analysis in accordance with ISO 13322-1 (static) and ISO 13322-2 (dynamic) captures two-dimensional particle projections and reports equivalent-circle diameters together with shape descriptors. It is valuable for verifying agglomeration, identifying elongated or irregular glass fragments, and cross-checking laser diffraction results.

Electrical Sensing Zone (Coulter Principle)

The electrical sensing zone method, covered by ISO 13319, provides true number-based particle counting in electrolyte suspension across roughly 0.6 µm to 800 µm depending on aperture size. It is well suited to counting sparse oversize particles and verifying fine-particle concentrations in glass slurries.

Sieve Analysis and Air-Jet Sieving

Dry sieving and air-jet sieving, conducted per ISO 2591-1 with test sieves conforming to GB/T 6003 or ASTM E11, are used for coarser glass cullet and frit fractions, typically above 20–45 µm. Sieve residue determination is a simple, robust check for oversize contamination and is often specified as an incoming inspection criterion.

Sedimentation Methods

Gravity and centrifugal sedimentation based on ISO 13317 determine Stokes diameters and are applicable to fine, dense glass powders where sedimentation behavior is itself process-relevant, such as in slurry formulation and tape casting.

Surface Particle Inspection of Substrates

Finished substrates are evaluated with laser surface-scanning inspection systems, dark-field optical microscopy, and automated defect review tools that count and classify surface particles by size. Scanning electron microscopy with energy-dispersive spectroscopy can subsequently identify particle composition, distinguishing glass chips, fibers, and inorganic residues from organic contamination.

Applications

Substrate glass particle size testing supports quality assurance and process development across multiple industries:

• Display glass substrate manufacturing: raw material qualification, batch release, and melting process optimization for TFT-LCD and OLED substrate production
• OLED and display encapsulation: PSD control of sealing glass frits and pastes to ensure hermetic, low-porosity seal lines
• Electronic packaging and ceramics: glass frit and glass-ceramic powder characterization for thick-film circuits, insulating layers, and hermetic packages
• Photovoltaics: sealing glass and cover glass powder quality control for module encapsulation
• Incoming inspection: verification of supplier conformity by panel manufacturers and downstream users, often through accredited third-party laboratories
• Research and failure analysis: correlation of granulometry with defects, sintering behavior, and field failures during product development

FAQ

What particle size range can be measured for substrate glass?

Using a combination of laser diffraction, dynamic light scattering, image analysis, and sieving, particle sizes from a few nanometers up to several millimeters can be characterized. For most glass frits and powders, the critical range lies between 0.1 µm and 100 µm, which laser diffraction covers with excellent precision.

Which standards govern substrate glass particle size testing?

Commonly referenced standards include ISO 13320 (laser diffraction), ISO 22412 and GB/T 19627 (dynamic light scattering), ISO 13322 (image analysis), ISO 13319 (electrical sensing zone), ISO 13317 (sedimentation), ISO 2591-1 (sieving), together with GB/T 19077 and GB/T 6003 for laser diffraction and test sieves. Data presentation generally follows ISO 9276.

How much sample is required for testing?

Wet laser diffraction typically requires between 0.1 g and 1 g of glass powder per measurement, while dry dispersion may consume 1 g to 5 g. Image analysis and surface inspection requirements depend on the statistical confidence needed. In all cases, representative sampling from the bulk lot is essential for valid results.

Wet or dry dispersion—which is appropriate for glass powders?

Wet dispersion in a suitable carrier liquid with controlled ultrasonication is preferred for fine glass powders, because it breaks up weak agglomerates and gives a stable measurement. Dry dispersion is acceptable for coarser, free-flowing frits but must be validated to avoid particle fracturing or incomplete deagglomeration.

Why is agglomeration control important in glass powder measurement?

Glass particles, especially submicron fines, tend to agglomerate through electrostatic and van der Waals forces. If agglomerates are not dispersed before measurement, the reported distribution shifts coarser and false oversize fractions appear, leading to incorrect acceptance or rejection decisions.

How are results reported and how is traceability ensured?

Reports typically include the full distribution table, D-values, mean diameters, span, dispersion conditions, and instrument verification status. Measurements performed by accredited third-party laboratories in accordance with ISO 13320 and related standards provide traceable, mutually recognized results suitable for supplier qualification and regulatory documentation.